1. Field of the Invention
The present invention relates to a technique for processing a substrate, such as a semiconductor wafer, by supplying gases to it, and to a unit for supplying the gases.
2. Background Art
In semiconductor device production processes, processing of a substrate such as a semiconductor wafer (hereinafter referred to as a wafer), e.g., etching or CVD, is conducted by placing a substrate in a processing vessel, and injecting, onto the substrate, process gases from a gas supply unit called a gas shower head, set in parallel with the substrate.
On the other hand, the recent trend toward small-sized, higher-density patterns has brought to some processes such a problem that patterns formed on substrate surfaces by the processes are apt to be non-uniform in size. For example, in a process of making a line-shaped gate electrode for a transistor by etching a gate electrode material layer covered with a resist mask, it is not easy to ensure a high selective etching ratio, and the resist mask disappears before the etching of the gate electrode material layer has been completed. For this reason, a technique for etching a gate electrode material layer by the use of silicon nitride film (SiN film) as a hard mask is now examined.
However, lines made in SiN film by etching have such a strong tendency that a curve of the within-film distribution of the line widths is convex. In other words, the line widths tend to be greater in the center of the film than at the edge. Since a depositing material is easily deposited on SiN film, the non-uniformity of depositing gas distribution on a substrate surface readily affects the amount of a material to be deposited on the sides of the lines. On the other hand, since it is more difficult to exhaust a gas from the center of a wafer than from the edge, and the gas pressure is slightly higher in the center of a wafer than at the edge, the amount of the deposit is greater in the center of a wafer than at the edge, and this difference in the amount of the deposit is considered to affect the within-wafer distribution of line widths greatly.
For example, in the process of etching SiN film 103 lying under a photoresist mask 101 and SiO2 film 102, as shown in FIG. 11(a), by the use of a plasma of a process gas containing, for example, CH2F2 gas serving as a depositing gas and oxygen gas serving as an etching gas, as shown in FIG. 11(b), permissible variations in line size D are in the range of 10 nm or less. Lines not only in a high-line-density area of a wafer 100, e.g., an area in which the ratio of metal lines to insulating layers between them is about 1:1, but also in a low-line-density area of a wafer 100 that has so far been allowed to have relatively large variations in line size D, e.g., an area in which the above-described ratio is 1:2 or more, are required to have sizes D that fall in the above-described range of permissible variations.
The gas supply unit described in Patent Document 1 can supply a gas to the center and edge of a wafer separately, so that it is possible to make the depositing gas feed rate per unit area greater at the edge of the wafer than in the center. However, since the flow rate of an etching gas to be supplied to the edge of the wafer also becomes higher, even if a film is deposited in an increased amount, the film is etched in an increased amount. Roughly speaking, therefore, it is impossible to increase the amount of the deposit at the edge of the wafer, and also to improve the within-wafer distribution of line sizes.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-723 ((0052)-(0054)) (U.S. Patent No. 2005-29369)